The mechanical links, unlike for example with bonded or welded links, are links that can be disconnected.
One method generally used to produce mechanical links between panels or structures made from composite materials involves using several metallic interface parts, each attached to the composite material in a discontinuous manner and using mechanical attachment means such as screws, pins, bolts or others.
Although these solutions appear optimised, this is actually not the case, as they do not provide for the homogenous passage of forces over the entire wall of the composite structure.
This generates local overflows creating the risk of a progressive breakage appearing on the zones subjected to higher stresses known as the peel-off phenomenon.
Furthermore, with regard to mechanical analysis, these local overflows are difficult to quantify, which is detrimental to the reliability and optimisation of the structure, with the link produced therefore being difficult to guarantee.
Today, wind turbines are designed with blades measuring several tens of meters in length. Such blades clearly create many technical problems in addition to logistics problems. The transportation of such structures, and more particularly over land, is not easy.
Wind turbine blades are made from composite materials, which enable these moving parts to keep a reduced weight, which in turn reduces the attachment forces to be transferred.
These blades are generally made from several sections so as to simplify their transport and in particular their transport by lorry.
A problem therefore arises involving the mechanical assembly of composite parts within the perspective of a mechanically optimised solution in terms of mass, in particular in the event where the forces to be passed by the link are significant and complex, the link being required to resist significant mechanical, fatigue and environmental stresses with very high reliability.
Documents EP 1 584 817, EP 1 878 915 and WO 01/48378 describe links for sections of wind turbine blades made by several metallic parts.
With regard to document EP 1 878 915, the link uses metallic rods inserted into the walls of the central casing and bonded in place.
In document WO 01/48378, the links are distributed along the skin of the blade and in document EP 1 584 817, separate ties connected to the central casing of the sections are attached together and supported by elements covering the gap between the blades.
These examples of embodiment use discontinuous mechanical attachment means.
Furthermore, known methods exist for the calculation of metal/composite pinned links, even if such a calculation is fairly complex due to the number of phenomena that must be taken into account and in particular: the transfer of forces between the composite material and the pins, the shear within the pins, the peening of the composite material, the tensile strength of the composite material and/or of the metal, and the scribing axis.
It should be noted that the known methods for calculating force transfers are very approximate, as they simplify the assembly geometry by representing the latter in the form of a single shear which enables the use of the shear calculation method known as the Huth method.
Due to this approximation, classic methods do not allow for the exhaustive parameterisation of the links.
In particular, the Huth model does not allow for geometrically non-symmetrical links to be studied, which limits its field of use.
Moreover, the calculation formulae associated with this model, in particular for calculating shear within pins, brings about generalisation difficulties for multi-shear, with this model not at all being adapted to suit the latter.
Document FR 2 675 563 describes a method for linking a tube made from composite material with a tubular metallic part.
In this method, the metallic part is attached to the composite material both by bonding and by mechanical attachments, which results in the possibility of passing very high mechanical flows. This method, for which optimisation of the pin link depends on the orientation of the winding wires, is however specific to wound tubes and therefore to a specific method of manufacturing the composite material.
This concept does not directly apply to composite parts such as sections of wind turbine blades, which are not generally manufactured by winding, and therefore the fibre orientation of which is set by the conditions for use of these blades.